Application package for powder coatings

ABSTRACT

Methods and systems for coating metal substrates are provided. The methods and systems include a powder coating composition comprising a polymeric binder and an application package. The application package includes at least one antistatic component and at least one post-blended component. Use of the application package reduces back ionization and faraday cage effects during electrostatic application. The described methods provide coatings with optimal surface smoothness and edge coverage.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of pending International ApplicationNo. PCT/US2013/030506 filed on Mar. 12, 2013, which claims benefit ofU.S. Provisional Application No. 61/712,110 filed on Oct. 10, 2012, U.S.Provisional Application No. 61/613,647 filed on Mar. 21, 2012 and U.S.Provisional Application No. 61/642,578 filed on May 4, 2012, each ofwhich is incorporated herein by reference in its entirety.

BACKGROUND

Powder coatings are solvent-free, 100% solids coating systems that havebeen used as low VOC and low cost alternatives to traditional liquidcoatings and paints.

Powder coating of metal parts is a common practice. It is difficult,however, to coat certain parts of a metal substrate, including edges andcorners, for example, to obtain a uniform coating using typical powdercoating processes, and edge corrosion is a common problem. Typically,when powder coatings are applied to metal parts, powder particles do notflow into corners, over edges, or into other recessed areas due tofaraday cage effects. Moreover, when a powder coating composition isapplied to a surface already coated with an uncured or non-gelledpowder, surface charge tends to build up on the surface, leading tosignificant back ionization. This may result in undesirableelectrostatic mixing of layers in the coating, and a consequentreduction in smoothness and gloss. Conventional systems that attempt toovercome faraday cage effects and back ionization problems typicallyrequire line modifications, modifications to the spray apparatus orchanges to the physical characteristics of the powder compositionitself. However, such methods typically lead to process inefficiency,increased costs and delay.

From the foregoing, it will be appreciated that there is a need foreffective powder coating of metal parts, where faraday cage effects andback ionization are eliminated, and where the coating demonstratesexcellent performance characteristics, such as excellent corrosionprotection, including at the edges, and optimal surface smoothness orgloss, without significant equipment or line modification.

SUMMARY

The invention described herein includes methods and systems for coatingmetal substrates using one or more powder compositions. In anembodiment, the methods include providing a metal substrate and applyinga first powder coating on the substrate. A second powder coating is thenapplied on the first powder coating, where the second powder coatingincludes at least one polymeric binder resin and an additive package.The two coatings are then cured simultaneously to produce a coating withgood corrosion resistance, including at the corners, edges and otherrecessed areas, and optimal surface smoothness or gloss.

In another embodiment, the present invention includes systems forcoating a metal substrate. The system includes at least one powdercomposition with an application additive. When the powder composition iselectrostatically applied over a non-gelled powder coating on asubstrate, the resultant coating has optimal corrosion resistance andsurface smoothness or gloss.

In another embodiment, the present invention includes methods forcoating a metal substrate, where the methods include providing at leastone polymeric binder resin, adding an application package to form amixture, and extruding the mixture. The methods further includepulverizing the mixture to form a powder coating composition.

In yet another embodiment, the present invention includes methods andsystems for coating a metal substrate, where the methods and systemsinclude providing at least a first powder composition, which includes anapplication package. The application package includes an antistaticcomponent and a highly dispersed component.

The details of one or more embodiments and aspects of the invention areset forth below. Other features, objects, and advantages of theinvention will be apparent from the description and from the claims.

Selected Definitions

Unless otherwise specified, the following terms as used herein have themeanings provided below.

The term “on”, when used in the context of a coating applied on asurface or substrate, includes both coatings applied directly orindirectly to the surface or substrate. Thus, for example, a coatingapplied to a primer layer overlying a substrate constitutes a coatingapplied on the substrate. Additionally, the term “metal substrate,” asused herein refers to substrates that are untreated, unprimed orclean-blasted, and also to surfaces that have been primed or pretreatedby various methods known to those of skill in the art.

The term “smoothness”, as used herein, refers to the specular gloss orlight reflectance from a powder-coated surface. It is typically obtainedby comparing the specular reflectance from a coated sample to thespecular reflectance from a black glass standard. As used herein,smoothness may be expressed by any means known to those of skill in thepowder coating art, including visual standards developed by the PowderCoating Institute. Under this standard, a visual scale of tenpowder-coated panels, graded from 1 (high roughness/orange peel) to 10(very smooth, high gloss finish) is used. To determine relativesmoothness, a powder-coated sample is visually compared with thestandard panels, and a smoothness grade is assigned by judging whichstandard panel is closest to the sample. In the alternative, surfacesmoothness may be expressed as 20-degree or 60-degree gloss measuredusing ASTM Method D523. Additionally, smoothness may be assessed byother methods known in the art, including for example, monitoring thedistinctness of the image (DOI), where the reflection of a powder-coatedsample in each of the 10 PCI test panels is photographed, and the speedof a beam of light reflected from the surface is measured by a specialinstrument. Surfaces that reflect an image perfectly have DOI value of100, while surfaces with little or no image clarity have DOI value of 0.The method used to determine smoothness will typically depend on theultimate end use for the powder-coated substrate.

The term “faraday cage effect,” as used herein, refers to a phenomenonobserved during electrostatic application of a powder coating.Specifically, a faraday cage is an electrostatic enclosure resultingfrom the pattern of field lines generated between the electrode on thespraying apparatus and the grounded substrate. The difference in voltagebetween the two surfaces induces a strong electric field between thespraying apparatus and the substrate. Consequently, charged powderparticles follow the resultant field lines and do not cover corners,edges and other recessed areas. Faraday cage penetration refers to theability of certain powder particles to travel through holes in theelectrostatic enclosure or faraday cage, and coat the substrate.

As used herein, the term “back ionization” refers to a phenomenon thatoccurs when a powder coating is applied over a non-gelled surface,typically a first powder coating that has not been fully cured. As morepowder is applied, the surface becomes saturated with charge and powderparticles are repelled from the surface, with resultant electrostaticmixing of powder layers, leading to non-uniform coating and film build.

The term “transfer efficiency,” as used herein, refers to the percentageof powder particles or coating solids leaving the spray apparatus thatare actually deposited on the substrate. Back ionization and faradaycage effects reduce overall transfer efficiency.

Unless otherwise indicated, the term “polymer” includes bothphotopolymers and copolymers (i.e., polymers of two or more differentmonomers).

The term “comprises” and variations thereof do not have a limitingmeaning where these terms appear in the description and claims.

The terms “preferred” and “preferably” refer to embodiments of theinvention that may afford certain benefits, under certain circumstances.However, other embodiments may also be preferred, under the same orother circumstances. Furthermore, the recitation of one or morepreferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from the scopeof the invention.

As used herein, “a,” “an,” “the,” “at least one,” and “one or more” areused interchangeably. Thus, for example, a coating composition thatcomprises “an” additive can be interpreted to mean that the coatingcomposition includes “one or more” additives.

Also herein, the recitations of numerical ranges by endpoints includeall numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2,2.75, 3, 3.80, 4, 5, etc.). Furthermore, disclosure of a range includesdisclosure of all subranges included within the broader range (e.g., 1to 5 discloses 1 to 4, 1.5 to 4.5, 1 to 2, etc.).

DETAILED DESCRIPTION

Embodiments of the invention described herein include methods andsystems for powder coating a metal substrate, including at the corners,edges and other recessed areas. The methods include steps for applyingat least one powder composition to a substrate, wherein using anapplication package or additive reduces faraday cage effects and backionization, and increases transfer efficiency when a powder coating isapplied electrostatically over a non-gelled surface or powder.

Accordingly, in some embodiments, the present invention provides methodsor systems for coating a substrate, including at the corners, edges orother recessed areas, with a process that uses an application additiveto increase faraday cage penetration while limiting back ionization. Themethods described herein therefore eliminate the need to modify sprayequipment or spray lines in order to reduce faraday cage effects or backionization. Moreover, as the methods described herein provide optimalcoverage for the corners, edges, and other recessed areas of substrates,alternative mechanical methods to coat the corners or edges prior toapplying a powder coating are no longer necessary. Therefore, themethods described herein reduce the time, energy and cost of powdercoating a substrate, including at the edges, without compromisingcorrosion resistance or surface smoothness of the coating.

In an embodiment, the methods described herein include applying at leasta first powder composition to a substrate. The powder composition is afusible composition that melts on application of heat to form a coatingfilm. The powder is applied using methods known to those of skill in theart, such as, for example, electrostatic spray methods, to a filmthickness of about 25 to about 75 microns, preferably 35 to 65 microns.In an aspect, the powder composition is applied to either the clean(i.e., unprimed) or pretreated surface of a metal substrate, i.e., thepowder composition may be applied to a metal surface that is unprimed,that has been clean-blasted or e-coated, or a surface that has beenpretreated by various methods known to those of skill in the art. In apreferred aspect, the powder composition is applied over a first powdercomposition that has not been cured or is only partially cured, i.e. anon-gelled powder coating.

In an embodiment, the method described herein includes applying at leasta second powder composition to a substrate after at least a first powdercomposition has been applied. In an aspect, the second powdercomposition may have the same chemical composition as the firstcomposition, or it may be different. The first and second powdercompositions may be compatible or incompatible. The second powdercomposition is applied using methods known to those of skill in the art,such as, for example, electrostatic spray methods. The second powdercomposition may be applied at a reduced film build, as it is preferablyapplied over a non-gelled or uncured or partially cured coating of thefirst powder composition, to a film build of 20 to 40 microns,preferably 25 to 100, more preferably 50 to 75 microns. The optimaltotal thickness of the film formed by the powder composition, or byfirst and second powder compositions may be about 50 to 175, preferably85 to 135 microns (approx. 3.5 to 5.5 mil).

In an embodiment, the methods described herein include applying at leasta second powder composition after at least a first powder compositionhas been applied on the metal substrate. Powder compositions aretypically applied by electrostatic spray methods, where the particles ofthe powder composition are charged and the substrate is grounded orearthed. Powder particles are charged by either the interaction of theparticles with ionized air (i.e., corona-charged), or by friction (i.e.,tribo-charged). Application-related complications can arise duringelectrostatic spray, and film build and transfer efficiency are affectedby electrical field lines between the spray gun and the substrate, i.e.,faraday cage effects. These problems may be overcome by using fluidizedbed methods for coating a substrate, but these methods producesubstantially thicker coatings and are not feasible for variousend-uses.

A powder composition applied or sprayed onto a surface coated with anon-gelled powder is unlikely to produce gloss or surface smoothnesssimilar or equivalent to a powder composition sprayed directly on to ametal surface. Without limiting to theory, it is believed that when asecond powder composition is applied over a first non-gelled powder, thebuild-up of charged powder particles on the surface leads to backionization, and powder particles begin to lift off the surface,resulting in non-uniform film build. Moreover, faraday cage formation istypical, and inhibits coating at the edges, corners, or other recessedareas of the substrate, leading to a non-uniform film and/or reducedsurface smoothness. Without limiting to theory, to overcome faraday cageeffects when electrostatically applying powder coatings to surfaces withcorners, edges, sharp surfaces or other recessed or non-planargeometries, it is necessary to decrease faraday cage formation orincrease the number of particles that can penetrate the faraday cage.

Conventionally, faraday cage effects are decreased by reducing theoutput of the application tool, such as a spray gun, for example, toincrease holes in the faraday cage, such that more powder particlespenetrate the faraday cage and are deposited. However, such methodsrequire reducing the line speed, and an overall decrease in transferefficiency. Alternatively, faraday cage penetration can be increased byaltering the characteristics of the powder particles, as described forexample, in U.S. Pat. No. 6,130,281. In general, smaller particles havethe best penetration but demonstrate lower transfer efficiency.Therefore, in order to coat an article or substrate with optimal filmbuild and smoothness, including at the edges, corners or other recessedareas, it is preferable to modify the application characteristics of thecoating composition.

Accordingly, the methods and systems described herein include a powdercoating composition and an application package. The application packagedescribed herein helps reduce faraday cage effects and back ionizationwithout significant modification of powder characteristics orapplication equipment. The application package described herein allowsan article or substrate already coated with a non-gelled powdercomposition to be sprayed or coated with a second powder composition atoptimal film thickness, and contrary to expectations in the industry,provides surface smoothness or gloss at least equivalent to powdercoatings applied directly to metal.

In an aspect, the methods described herein produce optimal surfacesmoothness. The methods described herein produce surface smoothness onthe PCI scale of at least 4, preferably at least 5. Measured as20-degree gloss or 60-degree gloss, the methods described herein producerelative surface smoothness of about 25 to 90%, preferably above 60%,and more preferably above 80%. Typically and preferably, the smoothnessof the surface will be determined by the desired end use for thepowder-coated metal substrate.

In an embodiment, the powder composition includes at least one polymericbinder. The powder composition may also optionally include one or morepigments, opacifying agents or other additives.

Suitable polymeric binders generally include a film forming resin andoptionally a curing agent for the resin. The binder may be selected fromany resin or combination of resins that provides the desired filmproperties. Suitable examples of polymeric binders include thermosetand/or thermoplastic materials, and can be made with epoxy, polyester,polyurethane, polyamide, acrylic, polyvinylchloride, nylon,fluoropolymer, silicone, other resins, or combinations thereof.Thermoset materials are preferred for use as polymeric binders in powdercoating applications, and epoxies, polyesters and acrylics areparticularly preferred. If desired, elastomeric resins may be used forcertain applications. In an aspect, specific polymeric binders or resinsare included in the powder compositions described herein depending onthe desired end use of the powder-coated substrate. For example, certainhigh molecular weight polyesters show superior corrosion resistance andare suitable for use on substrates used for interior and exteriorapplications. In a preferred aspect, the polymeric binder is apolyester, preferably an isophthalic-based polyester crosslinked withtriglycidyl isocyanurate (TGIC).

Examples of preferred binders include the following: carboxyl-functionalpolyester resins cured with epoxide-functional compounds (e.g.,triglycidylisocyanurate), carboxyl-functional polyester resins curedwith polymeric epoxy resins, carboxyl-functional polyester resins curedwith hydroxyalkyl amides, hydroxyl-functional polyester resins curedwith blocked isocyanates or uretdiones, epoxy resins cured with amines(e.g., dicyandiamide), epoxy resins cured with phenolic-functionalresins, epoxy resins cured with carboxyl-functional curatives,carboxyl-functional acrylic resins cured with polymeric epoxy resins,hydroxyl-functional acrylic resins cured with blocked isocyanates oruretdiones, unsaturated resins cured through free radical reactions, andsilicone resins used either as the sole binder or in combination withorganic resins. The optional curing reaction may be induced thermally,or by exposure to radiation (e.g., UV, UV-vis, visible light, IR,near-IR, and e-beam).

The methods and systems described herein include at least one powdercomposition with an application package. Suitable application packagesinclude a combination of various ingredients, including preferably anantistatic component, a highly dispersed component and the like. Theantistatic component is preferably an extrudable component, and thehighly dispersed component is preferably a post-blended additivecomponent.

In an embodiment, the antistatic additive is an extrudable component.Suitable extrudable antistatic components include, for example, salts ofalkali metals, salts of alkaline earth metals, quarternary ammoniumsalts, polymeric antistatic agents, surfactants, and the like. Suitablemetal salts for use as the antistatic component include salts of C₁-C₂₀monocarboxylic acids or C₂-C₂₀ dicarboxylic acids, salts of C₁-C₂₀sulfonic acids, organic acid salts of alkali metals or alkaline earthmetals, salts of hydrohalic acids, salts of inorganic acids, and thelike. Examples include, without limitation, salts of carboxylic acidssuch as formic acid, acetic acid, propionic acid, oxalic acid, succinicacid, etc., salts of sulfonic acids such as methanesulfonic acid,p-toluenesulfonic acid, etc., salts of organic acids such as thiocyanicacid etc., salts of hydrohalic acids, such as HCl, HBr, etc., and saltsof inorganic acids such as HClO₄, H₂SO₄, H₃PO₄, etc. Halide salts,acetates and perchlorates of alkali metals are preferred.

Suitable quarternary ammonium salts for use as the antistatic componentinclude inorganic and organic salts, i.e. salts formed by the reactionof a tertiary amine compound with an inorganic or organic acid.Inorganic quarternary ammonium salts include, for example, quarternaryammonium salts of HF, HCl, HBr, HI, HNO₃, H₂SO₄, H₃PO₄, H₃BO₃, HClO₄,and the like, as well as mixtures or complexes of these acids. Organicquarternary ammonium salts include, without limitation, compounds havingthe structure N(R)₄ ⁺X⁻, wherein each R is independently substituted orunsubstituted C₁-C₁₈ alkyl or aryl, and X is a anionic radical.Preferred organic quarternary ammonium salts include alkyl ammonium ordialkyl ammonium salts, more preferably methyl ethyl ammonium ordimethyl ethyl ammonium salts of organic acids, most preferably methylethyl ammonium or dimethyl ethyl ammonium salts of lauric acid, myristicacid, stearic acid, oleic acid, and the like. The anionic radical ispreferably C₁-C₁₀ alkyl sulfates, more preferably ethyl, propyl or butylsulfate, most preferably ethyl sulfate.

Suitable polymeric antistatic agents include thermoplastic resins withantistatic groups or blocks incorporated into the polymer chain, suchas, for example, block copolymers with polyolefins, polyether esteramides, and the like. Examples of block copolymers suitable for themethods described herein include, without limitation, polymers orcopolymers obtained by polymerization of one or more olefins with 2 to30 carbon atoms, preferably 2 to 10 carbon atoms, and more preferablypropylene or ethylene carbon atoms. Examples of polyether ester amidessuitable for the methods described herein include, without limitation,polyether ester amides made from Bisphenol A.

Suitable surfactants for use in the methods described herein include,for example, nonionic, anionic, cationic or zwitterionic surfactants.Examples of nonionic surfactants include, without limitation,polyethylene glycol type surfactants, polyhydric alcohol typesurfactants, and the like. Anionic surfactants include, for example,carbonates, sulfuric acid ester salts, sulfonates, phosphoric acid estersalts of alkali metals, higher alcohols, higher alkyl ethers,alkylbenzene, paraffin and the like. Examples of cationic surfactantsinclude quaternary ammonium salts such as alkyltrimethylammonium saltsand the like, as described above. Zwitterionic surfactants include, forexample, amino acid type surfactants, betaine type zwitterionic, and thelike. Surfactants may be used singly or in combination with othersurfactants or classes of surfactants. Preferably, the surfactant is ananionic surfactant, more preferably sulfonates, and most preferablyalkylbenzene sulfonates, alkylsulfonates, paraffin sulfonates, and thelike.

The methods and systems described herein include an application packagethat includes an antistatic component and a highly dispersed component.In an embodiment, the highly dispersed component is a post-blendedcomponent. The term “post-blended” refers to an active ingredient orcomponent added to the powder composition after extrusion but during orprior to the pulverization stage. Suitable post-blended components foruse in the methods described herein include, for example, waxes, metaloxides, metal hydroxides, mixtures thereof, and the like. Examples ofwaxes include natural waxes (carnauba, lanolin, beeswax, and the like),petroleum-derived waxes (paraffin and the like), polymer-based waxes(polyethylene wax, PTFE wax, polyamide-modified wax, and the like), andhydrocarbon wax (long chain (i.e., at least C₁₆) fatty acid orester-based waxes, and the like). Examples of metal oxides includehydrophobic silica, hydrophilic silica, aluminum oxide (i.e., alumina),and the like. Examples of metal hydroxides include aluminum hydroxide,and the like. Mixtures of metal oxide and metal hydroxide are suitablepost-blended components, as are wax-coated metal oxides and metalhydroxides. Preferably, the post-blended component is a highly dispersedfumed metal oxide, more preferably fumed alumina. Preferably, thepost-blended component is a highly dispersed fumed metal oxide with D50particle size of about 6 to 60 microns, preferably about 10 to 30microns, or agglomeration particle size of about 1 to 100 nm, preferably5 to 50 nm.

In a preferred embodiment, the methods and systems described hereininclude an application package that includes an antistatic component anda highly dispersed fumed metal oxide. The antistatic component ispreferably a pre-extrusion additive, while the fumed metal oxide ispreferably a post-blended component. The antistatic agent is preferablypresent in an amount of 0.1 to 3%, more preferably 0.05 to 2.5%, andmost preferably 0.01 to 2%, based on the total weight of the powdercomposition. The fumed metal oxide is preferably present in an amount ofup to about 2%, more preferably about 0.005 to 1.5%, and most preferablyabout 0.01 to 1%, based on the total weight of the powder composition.

The powder composition may optionally include other additives. Theseother additives can improve the application of the powder coating, themelting and/or curing of that coating, or the performance or appearanceof the final coating. Examples of optional additives which may be usefulin the powder include: cure catalysts, antioxidants, color stabilizers,slip and mar additives, UV absorbers, hindered amine light stabilizers,photoinitiators, conductivity additives, tribocharging additives,anti-corrosion additives, fillers, texture agents, degassing additives,flow control agents, thixotropes, and edge coverage additives.

In the methods described herein, the polymeric binder and antistaticagent are preferably dry mixed together, along with any optionaladditives, and then is typically melt blended by passing through anextruder. The resulting extrudate is solidified by cooling, thepost-blended component is added, and the extrudate is ground orpulverized to form a powder. Other methods may also be used. Forexample, one alternative method uses a binder that is soluble in liquidcarbon dioxide. In that method, the dry ingredients are mixed into theliquid carbon dioxide and then sprayed to form the powder particles. Ifdesired, powders may be classified or sieved to achieve a desiredparticle size and/or distribution of particle sizes. In a preferredaspect, the post-blended component is added during the pulverizationstep.

The resulting powder is at a size that can effectively be used by theapplication process. Practically, particles less than 10 microns in sizeare difficult to apply effectively using conventional electrostaticspraying methods. Consequently, powders having median particle size lessthan about 25 microns are difficult to electrostatically spray becausethose powders typically have a large fraction of small particles.Preferably the grinding is adjusted (or sieving or classifying isperformed) to achieve a powder median particle size of about 25 to 150microns, more preferably 30 to 70 microns, most preferably 30 to 50microns.

Optionally, other additives may also be used in the methods and systemsof the present invention. As discussed above, these optional additivesmay be added prior to extrusion and be part of the base powder, or maybe added after extrusion and/or during pulverization. Suitable additivesfor addition after extrusion include materials that would not performwell if they were added prior to extrusion; materials that would causeadditional wear on the extrusion equipment, or other additives.

Additionally, optional additives include materials which are feasible toadd during the extrusion process, but may also be added later. Theadditives may be added alone or in combination with other additives toprovide a desired effect on the powder finish or the powder composition.These other additives can improve the application of the powder, themelting and/or curing, or the final performance or appearance. Examplesof optional additives which may be useful include: cure catalysts,antioxidants, color stabilizers, slip and mar additives, UV absorbers,hindered amine light stabilizers, photoinitiators, conductivityadditives, tribocharging additives, anti-corrosion additives, fillers,texture agents, degassing additives, flow control agents, thixotropes,and edge coverage additives.

Other preferred additives include performance additives such asrubberizers, friction reducers, and microcapsules. Additionally, theadditive could be an abrasive, a heat sensitive catalyst, an agent thathelps create a porous final coating, or that improves wetting of thepowder.

Techniques for preparing low flow and high flow powder compositions areknown to those of skill in the art. Mixing can be carried out by anyavailable mechanical mixer or by manual mixing. Some examples ofpossible mixers include Henschel mixers (available, for example, fromHenschel Mixing Technology, Green Bay, Wis.), Mixaco mixers (availablefrom, for example, Triad Sales, Greer, S.C. or Dr. Herfeld GmbH,Neuenrade, Germany), Marion mixers (available from, for example, MarionMixers, Inc., 3575 3rd Avenue, Marion, Iowa), invertible mixers,Littleford mixers (from Littleford Day, Inc.), horizontal shaft mixersand ball mills. Preferred mixers would include those that are mosteasily cleaned.

The coating is optionally cured, and such curing may occur via continuedheating, subsequent heating, or residual heat in the substrate. Inanother embodiment of the invention, if a radiation curable powdercoating base is selected, the powder can be melted by a relatively shortor low temperature heating cycle, and then may be exposed to radiationto initiate the curing process. One example of this embodiment is aUV-curable powder. Other examples of radiation curing include usingUV-vis, visible light, near-IR, IR and e-beam.

The compositions and methods described herein may be used with a widevariety of substrates. Typically and preferably, the powder coatingcompositions described herein are used to coat metal substrates,including without limitation, unprimed metal, clean-blasted metal, andpretreated metal, including plated substrates and ecoat-treated metalsubstrates. Typical pretreatments for metal substrates include, forexample, treatment with iron phosphate, zinc phosphate, and the like.Metal substrates can be cleaned and pretreated using a variety ofstandard processes known in the industry. Examples include, withoutlimitation, iron phosphating, zinc phosphating, nanoceramic treatments,various ambient temperature pretreatments, zirconium containingpretreatments, acid pickling, or any other method known in the art toyield a clean, contaminant-free surface on a substrate.

The coating compositions and methods described herein are not limited toconversion coatings, i.e. parts or surfaces treated with conversioncoatings. Moreover, the coating compositions described herein may beapplied to substrates previously coated by various processes known topersons of skill in the art, including for example, ecoat methods,plating methods, and the like. There is no expectation that substratesto be coated with the compositions described herein will always be bareor unprimed metal substrates.

Preferably, the coated substrate has desirable physical and mechanicalproperties, including optimal edge coverage of sharp edges and surfacesmoothness. Typically, the final film coating will have a thickness of25 to 200 microns, preferably 50 to 150 microns, more preferably 75 to125 microns.

The following examples are offered to aid in understanding of thepresent invention and are not to be construed as limiting the scopethereof. Unless otherwise indicated, all parts and percentages are byweight.

The complete disclosure of all patents, patent applications, andpublications, and electronically available material cited herein areincorporated by reference. The foregoing detailed description andexamples have been given for clarity of understanding only. Nounnecessary limitations are to be understood therefrom. The invention isnot limited to the exact details shown and described, for variationsobvious to one skilled in the art will be included within the inventiondefined by the claims. The invention illustratively disclosed hereinsuitably may be practiced, in some embodiments, in the absence of anyelement which is not specifically disclosed herein.

EXAMPLES

Unless indicated otherwise, the following test methods were utilized inthe Example(s) that follow(s).

Gloss

The surface gloss of the coating is measured as 20-degree gloss or60-degree gloss using the procedure described ASTM D523 (Standard TestMethod for Specular Gloss).

Film Thickness Measurement

Dry film thickness is measured using a digital coating thickness gauge.On magnetic metal surfaces, the digital gauge operates viaeletromagnetic induction, and on non-ferrous metals, eddy currents areused to measure film thickness. These methods are standard and wellknown to those of skill in the art.

Example 1 Preparation of Coating Composition

Control formulations of a powder composition were prepared usingisophthalic-based polyesters crosslinked with triglycidyl isocyanurate(TGIC) to produce a composition with specific gravity of 1.34. Alteredformulations A and B were prepared in a similar manner, with about 0.5%of antistatic agent added prior to extrusion, based on the total weightof the powder composition. About 0.2% fumed aluminum oxide was added asa post-blend ingredient during pulverization of the Control formulationand Altered formulation A, but not Altered formulation B. The preparedpowder compositions were applied to cold-rolled steel panels over anexisting coating of a non-gelled powder on the panel surface.Smoothness, reported as 20-degree and 60-degree gloss, is shown inTable 1. Increased surface smoothness suggests reduced back ionizationand increased faraday cage penetration.

TABLE 1A Gloss Values Inside Faraday Cage (Control Formulation) ControlFormulation Over Non-Gelled Powder 20° Gloss Averages Line Speed LeadFace Lead Corner Middle Face Lag Corner Lag Face 18 f/min 58.5 49.5 62.553.0 61.0 60° Gloss Averages Lead Corner Middle Face Lag Corner Lag Face18 f/min 90.0 88.5 92.0 89.8 90.0

TABLE 1B Gloss Values Inside Faraday Cage (Altered Formulation A)Altered Formulation Over Non-Gelled Powder 20° Gloss Averages Line SpeedLead Face Lead Corner Middle Face Lag Corner Lag Face 18 f/min 86.5 66.574.0 54.0 84.0 60° Gloss Averages Lead Corner Middle Face Lag Corner LagFace 18 f/min 95.5 94.8 95.5 90.0 96.0

TABLE 1C Gloss Values Over Non-Gelled Powder (Altered Formulation B)Altered Formulation Over Non-Gelled Powder 20° Gloss Averages Line SpeedLead Face Lead Corner Middle Face Lag Corner Lag Face 18 f/min 77.0 58.567.5 55.3 82.0 60° Gloss Averages Lead Corner Middle Face Lag Corner LagFace 18 f/min 93.0 92.5 94.5 90.5 96.0

Example 2 Coating Thickness as Function of Faraday Cage Penetration

Panels were prepared as described in Example 1, and final cured filmthickness inside the faraday cage was measured and is shown in Table 2.An increase in cured film thickness, particularly at the corners,suggests increased faraday cage penetration by the powder particlesduring application.

TABLE 2A Measured Cured Film Thickness (Control Formulation) ControlFormulation Over Non-Gelled Powder Average film thickness (mil) LineSpeed Lead Face Lead Corner Middle Face Lag Corner Lag Face 18 f/min1.81 1.03 2.16 1.17 2.18

TABLE 2B Measured Cured Film Thickness (Altered Formulation A) AlteredFormulation Over Non-Gelled Powder Average film thickness (mil) LineSpeed Lead Face Lead Corner Middle Face Lag Corner Lag Face 18 f/min2.52 1.36 2.28 1.51 2.60

TABLE 2C Measured Cured Film Thickness (Altered Formulation B) AlteredFormulation Over Non-Gelled Powder Average film thickness (mil) LineSpeed Lead Face Lead Corner Middle Face Lag Corner Lag Face 18 f/min1.52 0.75 2.02 1.12 2.22

What is claimed is:
 1. A method for coating a substrate at one or morecorners, comprising: providing a metal substrate; applying a firstpowder coating composition; applying a second powder coating compositionover the previously applied non-gelled first coating composition, thesecond composition comprising: at least one polymeric binder resin; andan application package including an antistatic component; and a highlydispersed fumed metal oxide component with agglomeration particle sizeof 5 to 50 nm.
 2. The method of claim 1, wherein the application packagecomprises 0.1 to 2 wt % of a quaternary ammonium salt as an extrudableantistatic component; and 0.01 to 1 wt % of a fumed metal oxide as apost-blended highly dispersed component.
 3. The method of claim 1,wherein a cured film formed by the powder coating composition on acorner of the substrate has 20-degree gloss of at least
 60. 4. Themethod of claim 1, wherein a cured film formed by the powder coatingcomposition on a corner of the substrate has 60-degree gloss of at least60.